Para-xylene is a valuable chemical feedstock, which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C8 aromatic fractions from these sources vary quite widely in composition, but will usually be in the range of 10 to 32 wt. % ethylbenzene with the balance xylenes, being divided approximately 50 wt. % meta-xylene and 25 wt. % each of para-xylene and ortho-xylene.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption, or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C8 aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed while simultaneously minimizing any conversion of xylenes to other compounds.
Various methods for isomerizing xylenes and converting ethylbenzene have been used in the past. For example, U.S. Pat. No. 4,163,028 involves the isomerization of xylenes and the conversion ethylbenzene conversion using a catalyst comprised of zeolite, typified by ZSM-5, and a hydrogenation metal such as platinum.
U.S. Pat. No. 4,899,011 involves a process in which a C8 aromatic feed, which contains ethylbenzene and has been depleted in its para-xylene content, is contacted with a two catalyst system. The first catalyst dealkylates the ethylbenzene to benzene and ethane, while the second catalyst isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The volume of the first catalyst is no greater than one-half of the catalyst bed volume and, most preferably, not greater than one-third of the volume of the catalyst bed.
U.S. Pat. No. 5,516,956 involves a process in which a C8 aromatic feed, which has been depleted in its para-xylene content, is contacted with a two catalyst system. The catalyst used primarily to convert ethylbenzene comprises a zeolite that is selectivated with a selectivating agent containing silicon to improve its ethylbenzene conversion selectivity.
U.S. Pat. No. 6,008,425 involves a process for aromatics isomerization. The patent discloses a two catalyst system for the isomerization of xylenes and the conversion of ethylbenzene. The catalyst used primarily to convert ethylbenzene comprises zeolite bound by zeolite.
U.S. Pat. No. 6,576,581 involves a process for aromatics isomerization using a two component catalyst system. The catalyst used primarily to convert ethylbenzene comprises a non-zeolitic molecular sieve.
There is a need in the art to provide an improved process, which affords a high value product slate for ethylbenzene conversion and xylene isomerization.